Let a dislike be taken back and look tracks up by their pl8list path

An undislike event cancels a dislike in the taste profile, for the undo on the
dislike toast. GET /local/track?handle=&slug= returns one known track, so a link
into the app (/?t=handle/slug) can open on it.

backend/src/http/mod.rs

@@ -322,6 +322,7 @@ pub fn router(app: Arc<App>) -> Router {
         .route("/local/me", get(app::me))
         .route("/local/feed", get(app::feed))
         .route("/local/likes", get(app::likes))
+        .route("/local/track", get(app::track))
         .route("/local/artist", get(app::artist))
         .route("/local/event", post(app::event))
         .route("/local/follow", post(app::follow))